When running a zypper dup, the questions for dependency resolution and
for the licenses happen pretty quickly (< 30s), but what is it doing at
100% CPU for 10-15min(!) before starting to download the first package?

10-15 min? Wow, never seen that - what it's doing after having a solution
and downloading is calculating the installation order. You can see what
zypper is doing pretty clearly at /var/log/zypper.log